1. Technical Field of the Invention
The present invention relates to a mobile telecommunications network and, in particular, to the routing of an incoming call to a mobile station relocated from a first home location register to a second home location register.
2. Description of Related Art
Within the Global System for Mobile (GSM) Communication or the Personal Communications System (PCS), each mobile station is assigned a unique identification number known as a Mobile Station Integrated Services Digital Network (MSISDN) number. A MSISDN number is dialed whenever a caller wants to communicate with a particular mobile station. The telecommunications network determines, by analyzing a part of the dialed MSISDN number, the particular home location register (HLR) which is associated with that mobile station and which stores routing information identifying the mobile switching center (MSC) currently serving the mobile station. By retrieving and utilizing such routing information, the telecommunications network is able to locate the mobile station in response to an incoming call so that a call connection can be established between the incoming caller and the mobile station.
Mobile subscribers often relocate from a first service area served by a first HLR to a second service area served by a second HLR as they relocate from one city to another or one geographic area to another. By relocating, a particular mobile station's pre-existing subscriber agreement with the first HLR is terminated and a new subscription agreement with the second HLR must be established. However, a specific series of MSISDN numbers are pre-assigned to each HLR. Therefore, by relocating from one HLR to another, the mobile station accordingly must be assigned a new MSISDN number within the series pre-assigned to the new second HLR. Changing the assigned MSISDN number is a cumbersome process in part because the mobile subscriber has to take his mobile station in for service and also inconveniently notify all relevant parties of his new MSISDN number (directory number).
American Telephone & Telegraph (AT&T) has proposed the concept of the use of a location routing number (LRN) to alleviate some of the analogous problems associated with wireline terminals of subscribers who move from one region or network to another without changing their MSISDN numbers. In accordance with the LRN concept, a centralized database storing a network address representing the end office currently serving the wireline terminal is queried by a signal transfer point (STP) or a service switching point (SSP) within a Public Switched Telephone Network (PSTN) to route an incoming call to the correct end office. However, the above LRN concept is inapplicable for the mobile telecommunications environment because a mobile station is not physically attached to a single end office or mobile switching center (MSC). As a mobile station travels from one geographic area to another geographic area, a number of MSCs provide mobile service to the traveling mobile station. Therefore, implementing a centralized database with a network address representing a particular end office or MSC does not solve the above number portability problem within the mobile telecommunications environment.
A patent application titled "Routing An Incoming Call To A Ported Mobile Station Within A Telecommunications Network", filed by Guns-Shin Chien on Jun. 3, 1996 (hereinafter referred to as the Chien application), hereby incorporated by reference herein, discloses a system and method for maintaining a centralized database for storing data correlating a particular MSISDN with a network address representing the home Public Land Mobile Network (PLMN) and, more particularly, the home location register (HLR) within that particular PLMN currently serving the ported mobile station. Accordingly, when the centralized database is queried, the network address representing the current HLR serving the ported mobile station is returned.
Even though the Chien application enables the routing of an incoming call to a ported mobile station within a mobile telecommunications network, there are certain system characteristics which are not optimal for certain applications.
Since the telecommunications network cannot determine whether the mobile station has been ported or not just by analyzing the received MSISDN, even if only one number is ported out of a particular PLMN, all incoming calls intended for that particular PLMN have to be inefficiently queried by the centralized database. As an illustration, out of the 214-555-XXXX series, even if only one number is ported, incoming calls for each and every one of the 214-555-XXXX number series have to be queried by the centralized database. Even if a call is originated by a mobile station within the donor PLMN, the call must still be queried by the donor GMSC to the centralized database.
Furthermore, not all STPs, SSPs, and GMSCs connected to the donor PLMN or HLR have query capability. If an incoming signal is routed through an SSP not having query capability, the incoming signal will be incorrectly delivered to the donor HLR. Since the donor HLR no longer stores subscriber information pertaining to the ported mobile station, the routing of the incoming call fails.
Accordingly, there is a need for a mechanism to reroute incoming signals to the current PLMN serving a ported mobile station when such signals are received by the donor PLMN instead of being rerouted by a centralized database.